Referring to FIG. 1, a conventional metal-oxide-semiconductor field effect transistor (MOSFET) biosensor 1 includes a substrate 11, a gate terminal 12, a source terminal 13 and a drain terminal 14. The gate terminal 12, which is formed on the substrate 11, is disposed between the source and drain terminals 13, 14 and has a reactive layer 121 having receptors 15 immobilized thereon for specific binding with an analyte 16 in a liquid sample. By applying the liquid sample onto the reactive layer 121 and applying a bias voltage to the gate terminal 12, a monitored steady-state current obtained from the conventional biosensor 1 can be utilized to detect the analyte concentration in the liquid sample. However, the oxide layer in the conventional MOSFET biosensor may be easily contaminated by the ions in the liquid sample and results in adverse effect on the electrical property thereof. Furthermore, the conventional FET sensors are hard to directly detect analyte in liquid with high ionic strength, such as serum samples, due to a severe charge-screening effect.